1. Field of the Invention
The present invention relates to an apparatus for butt-joining starting and finishing ends of a sheet material with each other by drawing the both ends toward each other.
2. Description of the Related Art
Such a sheet material butt-joining apparatus is disclosed, for example, in Japanese Patent Examined Publication No. 55-14739. The apparatus disclosed in this publication comprises a stationary first base and a movable second base arranged above the first base and adapted to be moved vertically. A pair of bottom joining pawls extend along the starting and finishing ends of the sheet material and include bottom ends which are swingably supported by a top portion of the first base, and top ends provided with bottom joining sections which can be brought into contact with the starting and finishing ends of the sheet material, respectively, and which are inclined to approach to each other in an upward direction. A bottom energizing member serves to energize the bottom joining pawls in a rising direction. A pair of top joining pawls extend along the starting and finishing ends of the sheet material and include top ends which are swingably supported by a bottom portion of the second base, and bottom ends provided with top joining sections which can be brought into contact with the starting and finishing ends of the sheet material, respectively, and which are inclined so as to approach to each other in a downward direction. A top energizing member serves to energize the top joining pawls in the rising direction. A control member is provided, which can be situated at or retracted from an intermediate position between the starting and finishing ends of the sheet material, for maintaining a distance between said ends of the sheet material substantially constant at any position when the control member is situated at said intermediate position and contacted by said starting and finishing ends of the sheet material. A driving means serves to vertically move the second base by applying a vertically moving force thereto.
With such an arrangement of the known butt-joining apparatus, the starting and finishing ends of a sheet material are butt-joined with each other in the following manner. That is to say, the sheet material is first supplied to bottom joining pawls to bring the bottom of the starting end and the bottom of the finishing end into contact with the bottom joining section of one bottom joining pawl and that of the other bottom joining pawl respectively. In this instance, the starting and finishing ends of the sheet material is brought into contact with the control member which is situated at an intermediate position between these ends of the sheet material to keep the distance therebetween approximately constant at any position. Then, the control member is lowered and retracted from the intermediate position between the starting and finishing ends of the sheet material to form a gap with a predetermined width between the opposite ends of the sheet material. Then, the vertically moving means is operated to lower the second base and bring the top joining section of one top joining pawl into contact with the top of the starting end of the sheet material and the top joining section of the other top joining pawl into contact with the top of the finishing end of the sheet material, respectively. Thereby, the starting end of the sheet material is held vertically by the top and bottom joining pawls forming a pair and the finishing end of the sheet material is held by the other of the top and bottom joining pawls forming another pair. Because the second base continuously lowers even after the starting and finishing ends of the sheet material have been held, the top and bottom joining pawls are caused to move in the falling direction about the top and bottom ends, respectively, against the bottom energizing member. Thereby, the top and bottom joining sections of the top and bottom joining pawls approach to each other while holding the starting and finishing ends of the sheet material to butt-join these ends with each other. In this instance, in order to continuously lower the second base for accelerating the operation as described above, as soon as the top joining pawl contacts the starting and finishing ends of the sheet material and hold them together with the bottom joining pawl, the top and bottom joining pawls start moving in the falling direction to draw the starting and finishing ends of the sheet material to each other and urge them against the control member. Therefore, the control member arranged between the starting and finishing ends of the sheet material must be lowered and retracted from the intermediate position between the starting and finishing ends of the sheet material before they are urged against the control member, i.e., immediately before the top and bottom joining pawls hold the sheet material.
Thus, in the case of the above-mentioned known sheet material butt-joining apparatus, because the control member must be retracted from the intermediate position between the starting and finishing ends of a sheet material immediately before the top and bottom joining pawls contact the sheet material, the starting and finishing ends of the sheet material are not controlled until the top and bottom joining pawls hold the sheet material after the control member has been retracted. As a result, the starting and finishing ends of the sheet material are deformed and the distance between the starting and finishing ends changes depending on the position, and there may occur a problem that the joining strength and joining accuracy tend to fluctuate.